1. Technical Field
The invention relates to spectral analysis in far ultraviolet region.
2. Background Art
Recently, it is needed more and more to measure a very small change in the purity or characteristics of water in an aqueous solution, without changing the quality of water. For example, in a process for fabricating a semiconductor device, an aqueous solution is required to have high purity in correspondence to electrical resistivity of water at a level near theoretical limit. Specially purified water expected to have a particular effect by dissolving subtle gas such as ozone or hydrogen is also used recently.
In qualitative and quantitative analysis of water or soluble components therein, spectral analysis is used in various ways as a very powerful tool. Spectral analysis technique is divided mainly into ultraviolet and visible spectroscopy, near infrared spectroscopy and infrared spectroscopy with reference to wavelength region to be measured.
Especially, in near infrared spectroscopy, absorption spectra due to hydrogen bonds that are characteristic of water are observed noticeably in 800-1,400 nm region. For example, Japanese Patent laid open Publication H03-175341/1991 proposes to measure solute components with spectroscopy. Hydrogen bonds are formed between water molecules in water, and the state of hydrogen bond is changed very sensitively due to dissolved solutes into water. By studying a change in spectra, the dissolved components can be analyzed quantitatively. In concrete, when inorganic electrolytes are dissociated as ions in an aqueous solution, bonding state of water molecules themselves or a hydrogen bond between a water molecule near a hydrated ion and a water molecule in bulk water is affected due to disconnection or distortion of the hydrogen bonds. Then, near infrared spectra of an aqueous solution becomes different from that of pure water. By using a predetermined calibration curve, a concentration of an ion species can be measured quantitatively not from the absorption spectra of the ion species, but from that of water.
It is proposed recently to measure far ultraviolet spectra in order to determine a concentration of hydrated substance in an aqueous solution (Japanese Patent laid open Publication 2005-214863, and Applied Spectroscopy Vol. 58, No. 8 (2004) 910-916). This is based on a fact that far ultraviolet spectra of water are closely related to the state of hydrogen bond in water, similarly to the above-mentioned near infrared spectra. The absorption spectra of water has a peak around 150 nm of wavelength due to n→σ* transition, and the peak is shifted towards longer wavelengths due to an effect of electric field generated by water molecule itself and a hydrated ion. Therefore, a part of the absorption spectra is shifted into a wavelength region that can be measured with a conventional spectroscopy apparatus (a spectroscopy apparatus that does not need a vacuum system or nitrogen gas purge). By measuring the far ultraviolet spectra, an aqueous solution can be analyzed qualitatively, and solutes of very small concentrations can be measured quantitatively. This analysis using the far ultraviolet spectra of water is much sensitive qualitatively and quantitatively than that using the near infrared spectra. However, because the absorbance of water itself is very large in far ultraviolet region, the spectra can be measured only in a wavelength region longer than 180 nm which corresponds to a lower limit of transmission spectra.
Attenuated total reflection (ATR) spectrometry is explained here because it is used in the invention for measuring absorption spectra of a material having very large absorption. By using attenuated total reflection spectrometry, absorption in a sample can be measured due to penetration of light into the sample (evanescent wave) in the order of wavelength extending from a surface of an optical probe at which the light is reflected totally. The absorption spectra obtained is analogous theoretically to that measured with cell length of the order of wavelength. It is proposed in Japanese Patent laid open Publication S62-75230/1987 to measure a thick solution with an ATR probe. Synthetic quartz or sapphire is used as a material for fabricating the optical probe. In, for example, Japanese Patent laid open Publication H07-12716/1995, it is proposed to enhance the sensitivity of attenuated total reflection spectrometry itself.
The absorption observed in the absorption spectra of water in near infrared region is weak because it is ascribed to an inherently forbidden transition, so that very small concentration of a solute in an aqueous solution cannot be measured. Then, there is a need to measure a very small concentration of solute that cannot be measured with a significant difference in near infrared region. On the other hand, water has a large absorption peak around 150 nm of wavelength. By detecting a change in absorption spectra in far ultraviolet region, solutes in an aqueous solution can be measured qualitatively and quantitatively with a much higher sensitivity than in near infrared spectra. However, the absorption of far ultraviolet light of water is a large obstacle for spectral measurement of water or an aqueous solution in far ultraviolet region. If a substance other than water has strong absorption in far ultraviolet region, the absorption thereof is also a large obstacle for spectral measurement. It is to be noted that the prior art attenuated total reflection techniques mentioned above applied to infrared and visible regions cannot be applied to far ultraviolet region because transmittance is not sufficiently large in far ultraviolet region or the optical probe cannot cause total reflection at a surface in contact with a sample substance.